The Nu-SNS proposal - ORNL Physics Division - Oak Ridge ...

In addition to their effects prior to the formation of the supernova shock, charged-current
neutrino capture (and neutral-current inelastic neutrino scattering [47]) on heavy nuclei above the
shock can alter the entropy and neutronization of this infalling matter prior to its arrival at the
shock. It has been suggested that if sufficient energy is transferred to this matter to melt a
fraction of the nuclei, then the shock dynamics can be altered. Though this “pre-heating” of the
shock could help the shock, it could also hinder the shock because the melted nuclei produce a
higher pressure, reducing the Mach number of the shock. Potentially, these changes in the preshock
matter affect not only the shock propagation but also the thermodynamic conditions in the
post-shock convective region. Only with accurate neutrino-nucleus cross sections can we
gauge the full impact of these interactions on the supernova mechanism and begin to
answer the corresponding questions raised by Connecting Quarks with the Cosmos.
3.3 Supernova Nucleosynthesis
Neutrinos impact all phases of supernova nucleosysnthesis.
Supernova nucleosynthesis is commonly divided into several “processes”, each of which is
impacted by neutrino-nucleus interactions. (1) Explosive nucleosynthesis occurs as a result of
compressional heating by the supernova shock wave as it passes through the stellar layers. In the
inner layers of the ejecta, where iron group nuclei result from α-rich freezeout, interactions with
neutrinos alter the neutronization, changing the ultimate composition. (2) Neutrino
nucleosynthesis or the “ν” process occurs due to neutrino-induced nuclear transmutations in the
outer stellar layers followed by shock heating. (3) The rapid neutron capture or “r” process may
occur in the neutrino-driven wind that emanates from the proto-neutron star after the explosion is
initiated. The neutrinos both drive the wind and interact with the nuclei in it. Early phases of this
wind have also been suggested as the source of light p-process nuclei [48]. Thus, neutrinonucleus
interactions are important to all core collapse supernova nucleosynthesis processes.
3.3.1 Neutrinos and the α-Rich Freezeout
Neutrino interactions help determine the isotopic composition of the iron group ejecta.
One common property exhibited by recent spherically symmetric Boltzmann simulations [13,14]
is a decrease in the neutronization (which is equivalent to an increase in the electron fraction Y e )
of the inner layers of the ejecta due to neutrino interactions. This is a feature that current
parameterized nucleosynthesis models cannot replicate because they ignore the neutrino
interactions. The neutronization of the ejecta is important because galactic chemical evolution
calculations and the relative neutron-poverty of terrestrial iron and neighboring elements
strongly limits the amount of neutronized material that may be ejected into the interstellar
medium by core collapse supernovae [49]. Those previous multidimensional models for core
collapse supernovae that did produce explosions tended to greatly exceed these limits (see, e.g.,
[5,17,50]). To compensate, modelers have been forced to assume the fallback of a considerable
amount of matter onto the neutron star, occurring on a timescale longer than was simulated.
While the decreased neutronization seen in Boltzmann models reduces the need to invoke
fallback, it also makes any fallback scenario more complicated, since the most neutron-rich
material may no longer be the innermost.
As a result of neutrino-nucleus interactions, the nucleosynthesis products from future explosion
simulations (utilizing multi-group neutrino transport) will be qualitatively different, both in
ν-SNS Proposal 26 8/4/2005